In the absence of rapid therapeutic intervention, hemorrhagic shock (HS) results in a loss of intracellular energy charge and the dysfunction of critical organs. Impairment of the contractile function of the vascular smooth muscle, in particular, exacerbates organ injury and contributes to the development of therapy-resistant circulatory failure. In isolated vascular smooth muscle cells and vascular rings that peroxynitrite, a potent oxidant produced from nitric oxide (NO) and superoxide in HS, triggers an intracellular cascade culminating in cellular energetic failure. DNA strand-breakage mediated by peroxynitrite is recognized by a specific domain of the nuclear enzyme poly (ADP-ribose) synthetase (PARS). This results in the activation of PARS which initiates an energy consuming, inefficient repair cycle, with resultant depletion of dinucleotide pools, slowing the rate of glycolysis and mitochondrial respiration, reducing ATP synthesis. Thus, PARS acts as a terminal mediator of cellular energetic collapse. In addition, preliminary data show that basal PARS activity regulates the expression of the inducible isoform of NO synthase (iNOS) at the level of mRNA expression. Preliminary findings by these investigators show that inhibition of PARS improves vascular contractility, cellular energetic status and survival in rodent models of endotoxic shock. In the current proposal, the role of PARS in the pathophysiology of HS will be assessed. First, it will be established whether inhibition of PARS prevents the loss of vascular contractility during HS. This hypothesis will be tested by correlating blood pressure, regional macrovascular and microvascular perfusion, vascular contractility, DNA strand-breakage, PARS activity, and intracellular energetics, using rodent models of HS in which PARS activity is regulated by substrate inhibition or gene-targeted disruption. Second, the free radical and oxidant species responsible for the activation of PARS in HS will be identified. Specifically, the role of NO, peroxynitrite and hydroxyl radical will be studied. The activity of NO synthase will be inhibited by pharmacological tools and by gene-targeted disruption. Peroxynitrite will be scavenged by MnTBAP, a peroxynitrite scavenger compound. The relative contribution of hydroxyl radical will be investigated using mannitol. Third, the role of PARS in regulating iNOS expression will be investigated. The results of the present proposal will establish whether PARS will be a novel therapeutic target in the management of hemorrhagic shock.